CHAPTER XI. 



METABOLISM, NUTRITION, AND DIET. 



IT is not only necessary that the animal body should be supplied with 

 food in order that its natural functions may go on without interrup- 

 tion, but it is also equally requisite that the food should consist of proper 

 materials. It may be supposed that each kind of animal by instinct 

 keeps itself supplied with the substances which supply the needs of its 

 own metabolism the best, and it is a matter of every-day experience that 

 in the case of man, each endeavors to supply himself with food accord- 

 ing to the circumstances of his surroundings. We may therefore accept 

 such data as we can obtain from the observation of numerous examples 

 of such selection in the way of diet when we are in the act of drawing 

 up a diet-scale, relying upon such empiric knowledge alone, or, on the 

 other hand, we may proceed more scientifically, and endeavor to plan a 

 diet-scale from our experimental observation of the loss which takes 

 place in the body in the course of the twenty-four hours by the excreta. 

 If we do this we assume that the food is taken in to supply what is 

 generally called the waste of the tissues. The term is scarcely an accu- 

 rate one, but if we take it to mean in a restricted sense, what the 

 tissues and organs of the body give out to be eliminated by the excretory 

 organs in the course of the day, we may continue to use it. 



The food then may be supposed as intended to supply the place of 

 that which is given out by the body. But in the choice of a diet this is 

 not enough ; the food should be sufficient to supply such need without 

 waste and without unduly increasing the output of excreta, while at the 

 same time the body should be maintained in health, without increase 

 or loss of weight. 



These requisites of a diet scale then allow for wide alterations in the 

 amount of different kinds of foods under different circumstances. 



Careful analyses of the excreta, many of which we have already had 

 occasion to call attention to, show that they are made up, besides water, 

 chiefly of the chemical elements carbon, hydrogen, oxygen, and nitrogen, 

 but that they also contain, to a less extent, sulphur, phosphorus, chlorine, 

 potassium, sodium, and certain other of the elements. Since this is the 

 case it must be evident that to balance this waste, foods must be supplied 

 containing all these elements to a certain degree, but some of them, viz., 

 those which take a principal part in forming the excreta, in large amount. 



